1. Field of the Invention
The present invention relates to monitoring apparatus and more particularly to a method of monitoring the condition of a battery by measuring the internal resistance of the battery. The resistance of the battery is measured by connecting a first external load across the battery to obtain a reference voltage and then connecting a second external load across the battery to obtain a load voltage. The external loads can be the internal resistances of power transistors. The difference between the reference and load voltages can be employed to determine the internal resistance of the battery, which can then be compared with a predetermined warning value thereof such that a warning can be displayed if the power is lower than a predetermined level prior to starting the engine. The present invention further particularly relates to such a monitoring apparatus.
2. Description of Related Art
It is known that a driver has to start the engine of a motor vehicle before driving it. Also, for successfully starting the engine, there must be sufficient electric power stored in the battery. Typically, a battery has an approximate lifetime. However, factors such as ambient temperature, charging conditions and time, and load discharge all can adversely affect the lifetime of the battery. Thus, there is a difference between the practical lifetime and the approximate lifetime of any particular battery, and the difference is sometimes very large. Hence, in practice, there is no way for a driver (even an experienced one) to know the electric power level of the battery prior to starting the engine. Typically, only a motor vehicle repair shop technician is able to know the electric power level by means of a test device. However, the test device is somewhat bulky, thus prohibiting it from being carried by the motor vehicle. It is common that a driver finds the battery to be low only when it fails to start the engine. Alternatively, the battery may be already damaged but the driver is not aware before next start even it is successful in a particular start. This is not desirable and may even be dangerous since the driver may park his/her car in a remote area, mountain, or desert.
Advantageously, a battery power measuring device would be installed in a car as requisite equipment. But measuring the power of the battery may also consume the power of the battery. Hence, frequent power measurements are not desirable. Thus, it is especially desirable to accurately measure the power of a battery in a relatively short period of time and so as to consume a minimum amount of electric power. It is also important to measure the power of the battery while installed in the vehicle, which requires that the measurement technique employed be capable of providing accurate results even though unknown amounts of current may be flowing into and out of the battery at any time, due to the presence of battery chargers, alternators and the like on the one hand, and starting, lighting and accessory loads on the other.
The present application is relevant to commonly assigned U.S. Pat. Nos. 6,704,629 and 6,791,464 both entitled “Device for Monitoring Motor Vehicle's Electric Power and Method Thereof”, and to commonly assigned U.S. Pat. No. 7,212,006, entitled “Method and Apparatus for Monitoring the Condition of a Battery by Measuring its Internal Resistance.” The latter patent relates to a method and apparatus for monitoring the condition of a battery by measuring its internal resistance at two terminals of the battery by using a floating voltage V0 as its reference voltage to compare with a sampling voltage. However, the value of the floating voltage V0 itself is not stable as it will be affected by intermittent charging of the battery, up and down variation of the load at the battery, aging of the battery, and other factors.
Tsuji U.S. Pat. No. 6,072,300 relates to characterization of the individual batteries of a large set of batteries. Internal resistance is estimated from cell voltage. See Col. 5, lines 32-38.
Fakruddin U.S. Pat. No. 5,027,294 also characterizes battery condition based on measurements of voltage.
Huang U.S. Pat. No. 6,704,629, to the present inventor, measures battery condition in part by drawing a substantial current from the battery by connecting a significant load to it for a short period of time, as is part of the method of the present invention, but measures voltage only.
Arai U.S. Pat. No. 6,201,373 shows a circuit for measuring the state of charge (SOC) of a battery, not a battery condition evaluation device per se. Voltage and current are both sampled.
Hirzel U.S. Pat. No. 5,381,096 also relates to SOC measurement.
Satake U.S. Pat. No. 6,531,875 teaches estimating the open circuit voltage of a battery based on extrapolation from a series of measurements.
Disser et al. Pub. No. US 2003/0067221 A1 shows voltage regulator circuitry for automotive use.
Yokoo U.S. Pat. No. 5,828,218 shows a method for estimating residual capacity of a battery based on discharge current and voltage during discharge.
Munson U.S. Pat. No. 5,900,734 shows a battery monitoring system wherein the battery voltage is compared to a fixed reference value and an alarm is given when the battery voltage is less than the reference value.
Bramwell U.S. Pat. Nos. 5,721,688 and 6,097,193 discuss various methods of measuring the internal resistance and/or impedance of a battery, including application of a small AC signal to the battery and using a Wheatstone bridge or equivalent to measure the internal resistance. See col. 1, lines 40-48. Bramwell's claimed method includes the steps of measuring impedance of a battery by sourcing to or sinking from the battery a current of known magnitude at intervals while the vehicle sits. Col. 9, lines 18-50.
Turner et al. U.S. Pat. No. 6,249,106 shows a circuit for preventing discharge of a battery beyond a predetermined point. Yorksie et al. U.S. Pat. No. 3,852,732 is directed toward the same objective. Finger et al. U.S. Pat. No. 4,193,026 is directed to measuring the SOC of a battery by integrating a signal indicative of reduction of the terminal voltage below a threshold value.
Reher et al. U.S. Pat. No. 5,130,699 shows a device for monitoring a battery by measuring the terminal voltage at regular intervals, comparing the measured values to a predetermined value, and setting a flag in a shift register depending on the result. When a predetermined number of flags indicate an under voltage condition an alarm is given.
Sato et al. U.S. Pat. No. 5,193,067 discloses determining the internal impedance of a battery by measuring the voltage during discharge of a predetermined current, or by measuring the current during discharge at a predetermined voltage.
Slepian U.S. Pat. No. 5,764,469 shows disconnecting electronic equipment of a vehicle when the battery voltage falls below a predetermined level.
Huang U.S. Pat. No. 6,791,464, to the present inventor, shows evaluation of the condition of a motor vehicle's battery by monitoring the voltage across the battery during starting, while the starter provides a substantial load. The minimum voltage reached during starting can be compared to predetermined value to evaluate the condition of the battery.
Gollomp et al. U.S. Pat. No. 6,424,157 refers to the difficulty of measuring battery SOC from open-circuit voltage (OCV) because this requires that everything be disconnected. Gollomp instead teaches monitoring of the quiescent voltage (QV), e.g., measured at 30 minute intervals while the vehicle sits. Col. 9, lines 18-50. An alarm message can be given when QV falls below a predetermined point—Col. 11, lines 28-39. Gollomp also teaches monitoring of voltage and current during engine starting. See FIG. 6. This data is stored in memory, see Col. 12, lines 48-50, and used to determine dynamic internal resistance (IR) and polarization resistance (PR). Gollomp also teaches monitoring SOC and QV over time to determine when the battery won't be able to start the car; see FIG. 3, Col. 14, line 22-Col. 16, line 36. Gollomp also teaches storing the first IR value of the battery, or some subsequent one, for “future use”—e.g., determination of IR change over time. PR is similarly monitored over time; see Col. 17, line 12-Col. 18, line 35. The result is to give warning of incipient battery failure or some problem with connections or the like. These data can be monitored during successive starts; see claim 1.
Kchao U.S. Pat. No. 5,751,217 shows a method and circuit for assessing battery impedance, which is stated to be applicable only to fully charged batteries, see Col. 3, lines 49-55, and Col. 4, line 12, and which is intended to be incorporated in a battery charger. The apparatus of the invention is not limited to fully charged batteries and can be economically provided as a stand-alone unit.